Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS (complementary metal on semiconductor) with magnetic tunnel junction (MTJ) technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, and Flash. Furthermore, spin-transfer torque (STT) magnetization switching described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), has stimulated considerable interest due to its potential application for spintronic devices such as STT-MRAM on a gigabit scale.
Both field-MRAM and STT-MRAM have a MTJ element based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. One of the ferromagnetic layers has a magnetic moment that is pinned in a first direction while the other ferromagnetic layer has a magnetic moment that is free to rotate in a direction parallel or anti-parallel to the first direction. As the size of MRAM cells decreases, the use of external magnetic fields generated by current carrying lines to switch the magnetic moment direction of the free layer becomes problematic. One of the keys to manufacturability of ultra-high density MRAMs is to provide a robust magnetic switching margin by eliminating the half-select disturb issue. For this reason, a new type of device called a STT-MRAM was developed. Compared with conventional MRAM, STT-MRAM has an advantage in avoiding the half select problem and writing disturbance between adjacent cells. The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current traverses a magnetic multilayer in a current perpendicular to plane (CPP) direction, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic layer and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic free layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic free layer if the current density is sufficiently high, and if the dimensions of the multilayer are small.
For STT-MRAM to be viable in the 90 nm technology node and beyond, the ultra-small MTJ elements (also referred to as nanomagnets) must exhibit a magnetoresistive (MR) ratio that is much higher than in a conventional MRAM-MTJ which uses a NiFe free layer and AlOx as the tunnel barrier layer. Furthermore, the critical current density (Jc) must be lower than about 106 A/cm2 to be driven by a CMOS transistor that can typically deliver 100 μA per 100 nm gate width. A critical current for spin transfer switching (iC), which is defined as [(iC++iC−)/2], is generally a few milliamperes. The critical current density (Jc), for example (iC/A), is on the order of several 107 A/cm2. This high current density, which is required to induce the spin-transfer effect, could destroy a thin tunnel barrier made of AlOx, MgO, or the like. Thus, an important challenge for high density devices such as STT-MRAM on a gigabit scale is improving spin torque efficiency Eb/ic where Eb is the energy barrier between the two magnetic states shown in FIG. 1 and described below, and ic is the critical current needed to switch between the magnetic states. It is desirable to decrease ic (and its Jc) by approximately an order of magnitude so as to avoid an electrical breakdown of the MTJ device and to be compatible with the underlying CMOS transistor that is used to provide switching current and to select a memory cell.
Perpendicular magnetic anisotropy magnetic tunnel junctions (PMA-MTJs) are the building blocks that enable STT-MRAM and other spintronic devices. A preferred implementation is to employ a MTJ with a pinned ferromagnetic layer and free ferromagnetic layer separated by a tunneling oxide layer in a TMR configuration. As shown in FIG. 1, out of plane magnetization also known as perpendicular magnetic anisotropy (PMA) is depicted for pinned layer 10 with a magnetization 11 pointing in a z-axis direction or perpendicular to the film plane, and for free layer 20 with a magnetic moment 21 that is free to rotate either in a (+) or (−) z-axis direction. Tunnel barrier layer 15 is formed between the free layer and pinned layer. The free layer and pinned layer magnetizations are parallel or anti-parallel thereby establishing two different magnetic states. Thus, storage of digital information which is typically in the form of a “0” or “1” is provided by the direction of magnetization in the free layer.
When the free layer has a magnetization direction perpendicular to the plane of the film as in FIG. 1, the critical current (iC) needed to switch the magnetic element is directly proportional to the perpendicular anisotropy field as indicated in equation (1) where e is the electron charge, α is a Gilbert damping constant, Ms is the saturation magnetization of the free layer, h is the reduced Plank's constant, g is the gyromagnetic ratio, and Hkeff,⊥ is the out-of-plane anisotropy field of the magnetic region to switch, and V is the volume of the free layer:
                              i          c                =                              α            ⁢                                                  ⁢            e            ⁢                                                  ⁢            M            ⁢                                                  ⁢            s            ⁢                                                  ⁢            V            ⁢                                                  ⁢                          H                                                k                  eff                                ,                ⊥                                                          g            ⁢                                                  ⁢            ℏ                                              (        1        )            
The value Δ=kV/kBT is a measure of the thermal stability of the magnetic element where kV is also known as Eb or the energy barrier between the two magnetic states in FIG. 1, kB is the Boltzmann constant and T is the temperature. Thermal stability is a function of the perpendicular anisotropy field as shown in equation (2):
                    Δ        =                                            M              S                        ⁢            V            ⁢                                                  ⁢                          H                                                k                  eff                                ,                ⊥                                                          2            ⁢                                                  ⁢                          k              B                        ⁢            T                                              (        2        )            
The perpendicular anisotropy field of the magnetic element is expressed in equation (3) as:
                              H                                    k              eff                        ,            ⊥                          =                                            -              4                        ⁢                                                  ⁢            π            ⁢                                                  ⁢                          M              S                                +                                    2              ⁢                                                          ⁢                              K                U                                  ⊥                                      ,                    S                                                                                                      M                s                            ⁢              d                                +                      H                          k              ,              ⊥                                                          (        3        )            where Ms is the saturation magnetization, d is the thickness of the magnetic element, Hk,⊥ is the crystalline anisotropy field in the perpendicular direction, and KU⊥,S is the surface perpendicular anisotropy of the top and bottom surfaces of the magnetic element. The perpendicular anisotropy field of a magnetic layer (in the absence of strong crystalline anisotropy) is dominated by the shape anisotropy field (−4πMs), on which little control is available. At a given thickness, lower magnetization saturation decreases shape anisotropy and the spin-polarized switching current but also decreases thermal stability, which is not desirable. Therefore, one approach for improving spin torque efficiency is to decrease free layer volume and increase Ms.
Since the free layer must be able to withstand 400° C. temperatures during annealing processes necessary for CMOS fabrication, this high temperature requirement has led to the incorporation of refractory materials such as Ta, W, and Mo in the free layer to improve thermal stability. Unfortunately, these metals tend to induce the formation of a dead layer within the free layer. Moreover, free layer thickness is usually increased to recover the lost magnetic moment caused by inserting a refractory metal but this compensation works to lower the perpendicular anisotropy field and thermal stability according to equations (2) and (3) above. Thus, an alternative means of realizing improved thermal stability while maintaining high Ms and decreasing volume in a free layer is needed to provide a PMA-MTJ that will enhance STT-MRAM performance in state of the art devices.